it depends if you are thinking of going straight into another cleanse, or if you are stopping to eat for a while, and want to know how long you have to wait before you start one again. i've done some research on that, and the standard answer is to wait AT LEAST a month before starting another fast.
eneregeticnutrition.com reccomends waiting 4 weeks before repeating a fast. http://www.bbc.co.uk/health says not to repeat a fast for at least a month.
if you are going to go straight into another cleanse, then you can do that, provided you are getting enough nutrition and you haven't been cleansing for a really long time. (if you did a 40 day, i would go eat for a while before fasting again)
there are many cleanses out there, it is recommended that we actually do a liver/gaulbladder cleanse not too long after the Master-Cleanse (that isn't a fast, it only takes like a day, so you don't have to wait too long to do it.)
there are juice fasts, which do not get the toxins out quite as quickly as MC, but they are extremely beneficial, and they are easier to handle, can go longer, and supply you with all the nutrients you need. you could practically live on them.
i'd stop at a bookstore and get a book on cleansing and see what's right for you and your needs/lifestyle.